This invention relates to lightwave communications systems, and more particularly to multiwavelength lightwave communications systems.
In single wavelength long distance optical communications systems, fiber amplifiers, such as erbium-doped fiber amplifiers, are periodically spaced along the fiber path to compensate for the transmission losses that accumulate as the light traverses the transmission fibers and other optical components along the system. The gain of each amplifier in the cascade can match the signal loss in the portion of the transmission path that follows the previous amplifier in the cascade. It is known (see, e.g., C. R. Giles and E. Desurvire, "Modeling Erbium-Doped Fiber Amplifiers", IEEE Journal of Lightwave Technology, Vol. 9, No. 2, pp. 271-283, February 1991) that a cascade of saturated fiber amplifiers acts to self-regulate the signal power through the transmission system. Accordingly, the power output of the first amplifier in a cascade of saturated fiber amplifiers is duplicated at the output of all the subsequent amplifiers along the system.
There is currently considerable interest in building large, multiwavelength communications systems to support the envisioned high-capacity information networks of the future. Such multiwavelength systems will advantageously have increased signal carrying capacity. Furthermore, and even more significantly, the multiple wavelengths will be used for the purposes of signal routing. As in the single-wavelength optical communications systems, in multiwavelength systems cascades of optical amplifiers will be required to compensate for the losses that accumulate as the light traverses the transmission fibers, and the larger losses that the signals encounter from the optical switches and routers along the optical signal path.
The most significant technical obstacle standing in the way of such large, amplified multiwavelength communications systems is the nonuniform gain spectrum of fiber amplifiers. Although erbium-doped fiber amplifier gain spectra are typically flat within three decibels over a bandwidth of approximately 20 nm, these relatively modest gain nonuniformities in a single erbium-doped fiber amplifier will accumulate along a cascade, resulting in exponentially rising interchannel power variations.
Mathematically, each erbium-doped fiber amplifier provides gain, in dB, given by ##EQU1## where .sigma..sub.e (.lambda.) and .sigma..sub.a (.lambda.) are the emission and absorption cross-sections, n.sub.1 and n.sub.2 are the lower- and upper-state population densities, and l is the amplifier's length. The integrand of equation (1) exhibits a wavelength-dependence imposed by .sigma..sub.e (.lambda.) and .sigma..sub.a (.lambda.), which are in turn determined by the spectroscopic properties of erbium atoms in silicate glass. While this wavelength-dependence can be modified, via n.sub.1 and n.sub.2, by adjusting pumping and saturation levels, it cannot be eliminated. Equation (1) thus implies that wavelength-multiplexed channels traversing a chain of nominally identical amplifiers will develop an interchannel power spread that grows exponentially along the chain. Thus, as a multiwavelength network is scaled up in size, channels residing off the gain peak will fall towards power levels that are undetectable in the presence of receiver noise or amplifier-induced beat noise or both. Such behavior is in effect fundamental and inherent in the spectroscopy of erbium ions in silica glass, which is the only material system, so far, that has succeeded in providing practical optical gain for lightwave communications systems. Thus, power regulation that is noted above as being achievable with a cascade of saturated fiber amplifiers in a single wavelength optical system will not be effective in a multiwavelength optical system. Rather, in a multiwavelength system, the output of each conventional fiber amplifier will be regulated on a total power basis, and not on a channel-by-channel basis.
Various prior art approaches to this problem have been proposed. In a first approach (A. R. Chraplyvy, J. A. Nagel, and R. W. Tkach, "Equalization in Amplified WDM Lightwave Transmission Systems", IEEE Photon. Technol. Lett., Vol. 4, No. 8, pp. 920-922, August 1992) the transmitter power is selectively boosted for wavelengths that propagate weakly through the system. Such an approach may be effective in modest-size point-to-point links, but is not promising in networks, especially those with dynamically reconfiguring signal paths. A second approach (K. Inoue, T. Kominato, and H. Toba, "Tunable Gain Equalization Using a Mach-Zehnder Optical Filter in Multistage Fiber Amplifiers", IEEE Photon. Technol. Lett., Vol. 3, No. 8, pp. 718-720, August 1991) uses fixed filters to selectively suppress wavelengths that propagate too strongly. This approach has also achieved some success, but it is not adjustable in the event of component or amplifier-inversion-level variations. Moreover, it cannot be scaled to very large system sizes due to critical matching difficulties. In a third approach (S. F. Su et al, "Gain Equalization in Multiwavelength Lightwave Systems Using Acoustooptic Tunable Filters", IEEE Photon. Technol. Lett., Vol. 4, No. 3, pp. 269-271, March 1992) channel-suppression filters are embedded in servo-loops, one servo-loop being used per channel per amplifier. Although it is in principle effective, this approach is complex. Furthermore, this third approach imposes increased system losses that must themselves be compensated for by additional gain stages.
In co-pending patent application Ser. No. 056,098, filed May 5, 1993 pending, two of the co-inventors thereof also being co-inventors of the present invention, a scalable multiwavelength optical amplifier cascade is disclosed which presents a solution to this problem. As described therein, channel-by-channel self-regulation is achieved in an optical communications system by using a cascade of inhomogeneously broadened erbium-doped fiber amplifiers operated in gain-saturation as opposed to the conventional homogeneously broadened fiber amplifiers used in a single wavelength systems. Unlike in a conventional homogeneously broadened erbium-doped fiber amplifier in which all channels are coupled to a single collection of gain-giving erbium ions, in an inhomogeneously broadened erbium-doped fiber amplifier, each channel interacts with its own private set of erbium ions. It was thus recognized therein that a saturated cascade of inhomogeneously broadened fiber amplifiers will exhibit saturation-induced self-regulation of signal power on a channel-by-channel basis. Inhomogeneous broadening of the erbium-doped fiber amplifiers was achieved by cooling them to a temperature substantially below 0.degree. C., such as by placing them in a liquid nitrogen bath. The practicality of such an arrangement in an actual communications system, however, is at present undemonstrated. Although alternative methods of achieving inhomogeneous broadening of fiber amplifiers are feasible, further developments in applied physics are needed before other practical realizations are possible.
An object of the present invention is to provide in a multiwavelength lightwave communications system power self-regulation on a channel-by-channel basis with a minimum of optical component complexity and which can be implemented with maximum practicality.